Earlier work has indicated that heteropolysaccharides produced by the action of Xanthomonas bacteria on carbohydrate media have potential application as film forming agents, as thickeners in edible products, cosmetic preparations, pharmaceutical vehicles, oil field drilling fluids, fracturing liquids, similar compositions, and as emulsifying, stablizing, and sizing agents. Heteropolysaccharides, particularly, xanthan gum, have significant potential as mobility control agents in micellar polymer flooding. Xanthan gum has excellent viscosifying properties at low concentration, is resistant to shear degradation and exhibits only minimal losses in viscosity as a function of temperature, pH, and ionic strength. For these reasons, xanthan gum is an attractive alternative to synthetic polyacrylamides for enhanced oil recovery operations.
Fermentation of the inoculated medium with Xanthomonas organisms for 36-72 hours under aerobic conditions results in the formation of xanthan gum which is separated from other components of the medium by precipitation with acetone or methanol in a known manner. Because of time required to ferment each batch, the low biopolymer content of the fermented medium and the processing required for the recovery and purification of the product, xanthan is relatively expensive.
Other investigators have produced xanthan heteropolysaccharide by means of single stage or multistage "continuous" fermentation. In most instances, Xanthomonas campestris was grown in a medium containing dried distillers solubles (DDS) or other complex nutrient as a source of nitrogen and growth factors. There has been no instance of which we are aware, however, that xanthan was produced by continuous fermentation with Xanthomonas campestris or any organism of the Xanthomonas genus using a chemically defined medium containing glucose, ammonium chloride, mineral salts, and an organic supplement consisting of vitamins and/or amino acids.
It is well-known that continuous production of xanthan can be hampered by a tendency of the culture (Xanthomonas campestris) to change or degenerate after a specific number of turnovers; i.e., the time required to completely replace one volume of broth in the fermentation vessel, or the reciprocal of the dilution rate. Normally, 6 to 9 turnovers are the maximum that can be obtained before degeneration of the culture occurs. Coincident with degeneration, there is a decrease in xanthan viscosity, a loss in volumetric productivity of xanthan, i.e., grams of xanthan produced per liter of broth per hour, and the appearance of variant strains that no longer produce xanthan or else produce a poor quality of xanthan. It has been demonstrated that this phenomenon occurs when DDS is used as the complex nitrogen source whether in the whole form or as the water soluble extract.
The most pertinent publications of which we are aware are as follows.
1. P. Rogovin, et al, 1970, "Continuous Fermentation to Produce Xanthan Biopolymers: Laboratory Investigation", Biotechnol. Bioeng., XII, pp. 75-83.
2. K. W. Silman, et al., 1972, "Continuous Fermentation to Produce Xanthan Biopolymer: Effect of Dilution Rate", Biotechnol. Bioeng., XIV, pp. 23-31.
3. P. Rogovin, U.S. Pat. No. 3,485,719, "Continuous Production of Xanthan".
4. G. P. Lindblom, et al., U.S. Pat. No. 3,328,262, "Heteropolysaccharide Fermentation Process".
5. Netherlands patent application No. 7,612,448, "Method for the Production of Bacterial Polysaccharides".
6. "Production of Polysaccharides by Xanthomonas campestris in Continuous Culture", FEMS Microbiology Letters, 347-349 (1978) by I. W. Davidson.
7. Process for the Production of Xanthan Gum, British patent application No. 2,008,138A (Tate and Lyle, LTD).